TAMPA — Rita Girven was one of the Tampa Police Department's most prolific informers, helping investigators crack anywhere from 50 to 150 cases, according to the still-uncertain estimates of her attorneys and former police handlers.

Now, as she finds herself enmeshed in a corruption scandal that has led to a federal grand jury investigation involving at least three former Police Department employees, Girven's abundant cooperation could turn into a liability for law enforcement.

That's because new questions about Girven's credibility and relationship to the police force cast doubt on the integrity of investigations that relied on her assistance, experts say. In a worst-case scenario, prosecutors could be forced to review and possibly drop cases in which arrests were secured based on misleading information she supplied.

"If there's even one lie that is a substantive lie, that is the rotten apple that contaminates the entire barrel," said Michael Lyman, a professor of criminal justice at Columbia College of Missouri.

Girven is being held in jail without bail on charges including food-stamp fraud and grand theft. Authorities allege that former Tampa police Sgt. LaJoyce Houston illegally used Girven's food stamps to buy groceries.

Girven is also implicated in a federal investigation into LaJoyce Houston; Eric Houston, a longtime homicide detective and LaJoyce's husband; and Tonia Bright, who worked as a civilian community service officer. Both Houstons have been fired from the department, and Bright resigned after learning she was a target of the grand jury investigation.

Only LaJoyce Houston and Girven have been charged, in state court, for the alleged food-stamp scheme. But in a federal search-warrant affidavit filed last month, an investigator alleged that fraudulent tax refunds were sent to Girven's home address for people whose identities might have been stolen through police databases. The federal investigation is ongoing.

Girven's centrality to the alleged frauds could be problematic. Prosecutors are obligated to disclose information that could help defendants, including material that can be used to attack the credibility of witnesses and investigating officers — or informers who play a critical role in an arrest.

Even when they are not used as formal witnesses, informers' tips are often incorporated in the affidavits of officers seeking search and arrest warrants.

Tamara Lave, a professor at the University of Miami School of Law, said that if such warrants were granted primarily because of information Girven provided, they could be tossed out, leading to the suppression of evidence obtained through the warrants.

"Let's say that the only basis for the judge's decision is the alleged statements of the confidential informant," Lave said. "If that's the case, then that can make a difference."

Last week, Girven's defense attorney — Ralph Fernandez, a well-known Tampa lawyer whose previous clients include dozens of police officers and several sitting judges — filed a motion asking for the release of all police records of her work as an informer.

In addition to whatever help the documents provide Girven, Fernandez said, he expects they could also force a review of cases in which she played a role — a potential bonanza for other defense attorneys.

For police and prosecutors, Fernandez said, "No matter how you look at the Girven matter, it's a disaster."

Tampa police continue to decline to comment on or even confirm Girven's status as a confidential informer, which was first disclosed in the federal affidavit.

However, spokeswoman Laura McElroy said that in recent years the department, concerned about problems with Girven's credibility, did not use her as a sworn witness and verified her information through other sources before using it.

"She could not testify in court, and any information she provided was considered an investigative lead that had to be corroborated either by physical evidence or another witness," McElroy said.

She said the Police Department's attorneys are consulting with federal and state prosecutors on how to respond to Fernandez's request. Hillsborough State Attorney's Office spokesman Mark Cox said no review is under way of cases with which Girven was involved.

Based on his conversations with Girven, Fernandez said, he believes his client helped police on as many as 150 cases, including murders, burglaries and drug trafficking. That might seem an extravagant estimate, but it's not necessarily far-fetched in light of what has already been uncovered.

In November, Officer Rob Fannin told criminal investigators under oath that Girven had helped put "at least" 30 to 50 people in prison and had worked extensively with officers in her troubled East Tampa neighborhood. Fannin was questioned about his close relationship with Girven but cleared of wrongdoing, according to police.

McElroy said the cases Fannin referred to mainly involved Girven helping track down wanted suspects and would not be affected by the current scandal.

"Providing information that is helpful is a far cry from using her in a way that her credibility jeopardizes a case," McElroy said.

Peter Jamison can be reached at pjamison@tampabay.com or (813) 226-3337. Follow him on Twitter @petejamison.